A recent study from the University of Michigan (U-M) and Texas A&M yielded an unexpected finding - pollen grains may have an impact on rainfall.

Photo: Joseph Xu, Michigan Engineering Communications & Marketing

In a release, the University of Michigan highlighted pollen had largely been ignored by atmospheric scientists who study aerosols, which are particles suspended in the air that scatter light and heat, playing a role in cloud formation.

"The grains were thought to be too large to be important in the climate system, too large to form clouds or interact with the sun's radiation," said Allison Steiner, U-M associate professor of atmospheric, oceanic and space sciences.

"And also the large particles don't last in the atmosphere. They tend to settle out relatively quickly."

Sweeping dust off her porch one spring morning, Steiner wondered what happened to the grains in the air, and turned to the medical community for answers. There she found that pollen caused seasonal allergies which affected 10-20% of U.S. residents.

"When we were looking in the allergy literature we discovered that it's pretty well known that pollen can break up into these tiny pieces and trigger an allergic response," she said.

Smaller grains could have big implications. The research team set out to see if moisture could cause the pieces to break down.

"What we found is when pollen gets wet, it can rupture very easily in seconds or minutes and make lots of smaller particles that can act as cloud condensation nuclei, or collectors for water," Steiner said.

In a lab experiment at Texas A&M, researchers tested pollen from oak, pecan, birch, cedar and pine trees, as well as ragweed, which are the most common sources of win-driven pollen in the country.

The scientists soaked two grams from each source in pure water for an hour, and used an atomizer to make a spray of the moist pollen fragments to be placed in a cloud-making chamber in the laboratory of atmospheric science professor Sarah Brooks.

The team found that three different sizes - 50, 100 and 200 nanometers - of all six types began to pull in moisture and form clouds.

"Samples entering the cloud chamber are exposed to moist conditions representative of the relative humidity found in the atmosphere," Brooks said.

"If a sample is an effective cloud activator, droplets will rapidly grow on the sample fragments, forming large cloud droplets."

The release said the findings informed climate science and public health.

"What happens in clouds is one of the big uncertainties in climate models right now," Steiner said.

"One of the things we're trying to understand is how do natural aerosols influence cloud cover and precipitation under present day and future climate."

As next steps, the team plans to conduct similar studies in the field and, through computer simulations, model the potential feedback between plant life and the atmosphere.

"It's possible that when trees emit pollen, that makes clouds, which in turn makes rain and that feeds back into the trees and can influence the whole growth cycle of the plant," Steiner said.

A paper on the findings, titled "Pollen as atmospheric cloud condensation nuclei," has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

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